(1) Field of the Invention
The present invention relates to the amplification, with different pre-selected degrees of non-linearity, of the alternating and direct current components of a single input signal and particularly to differently amplifying the steady-state and varying components of a signal commensurate with the intensity of a monitored flame. More specifically, this invention is directed to a non-linear amplifier which exhibits different non-linearity for DC and AC components of an input signal commensurate with the intensity of a monitored flame and especially to a flame scanner which employs such amplifier. Accordingly, the general objects of the present invention are to provide novel and improved methods and apparatus of such character.
(2) Description of the Prior Art
While not limited thereto in its utility, the present invention is particularly well suited for use in a flame scanning system of a type employed to monitor the state of a burner in a fossil fuel fired furnace. Flame scanners are well known in the art and typically include a scanner head which comprises an optical flame sensing device such as a photocell. The photocell will provide a low level signal commensurate with the instantaneous intensity of the monitored flame. This low level signal, which has alternating (AC) and steady state (DC) components, is pre-amplified, to improve the signal-to-noise ratio thereof, and thereafter transmitted, typically by a cable, to signal processing electronics which provides visual and/or audible outputs commensurate with the state of the monitored flame. The scanner may include a light conductor, such as a fiber optic bundle, which is inserted into the furnace for the purpose of coupling the light resulting from the combustion of fuel in the region being monitored to the photocell.
In order to derive the desired information commensurate with flame state from the electrical signal produced by the photocell or other light responsive sensor, the signal transmitted to the processing electronics must have a high signal-to-noise ratio. This is particularly important since the AC component of the sensor output signal will typically be no higher than a few percent of the DC component. Also, the AC signal component must be amplified to a level where a signal which is reliably detectable will be present while, at the same time, the DC component of the sensor output signal must be limited to a level which will not saturate the processing electronics. In the prior art, in an attempt to address these conflicting signal component pre-processing requirements, it has been conventional practice to attempt to separate the AC and DC components of the sensor generated signal and then amplify the AC component after it has been separated from the DC component. Such separation prior to amplification has resulted in flame scanners which were comparatively complex and, of course, circuit complexity is to some degree inversely related to reliability.
For a general discussion of flame scanners of the general type to which the present invention relates, reference may be had to the "BACKGROUND OF THE INVENTION" portion of U.S. Pat. No. 4,322,723 which is assigned to the assignee of the present invention.